Multiplexed output two terminal photodiode array for imaging applications and related fabrication process

ABSTRACT

A detector array for an imaging system may exploit the different sensitivities of array pixels to an incident flux of low energy photons with a wavelength falling near the high end of the range of sensitivity of the semiconductor. The detector array may provide the de-multiplexable spatial information. The detector array may include a two-terminal multi-pixel array of Schottky photodiodes electrically connected in parallel.

RELATED APPLICATIONS

This application is a divisional of pending Ser. No. 12/895,081 filedSep. 30, 2010, the entire disclosure of which is hereby incorporatedherein by reference.

FIELD OF THE INVENTION

The invention relates generally to imaging systems. In particular, theinvention relates to a multi-pixel detector array for use in an imagingsystem and a method of fabricating the same.

BACKGROUND OF THE INVENTION

Detector arrays are used in a wide variety of imaging systems forindustrial as well as scientific applications. For example, detectorarrays have been recently widely used in nuclear medical imagingtechniques, such as Positron Emission Tomography (PET). In such anapplication 511 keV gamma rays emitted from a human body strikepixelated scintillators which create light in response to the receivedradiation. Each illuminated pixel of the pixelated scintillator isdetected by a respective photodiode of the detector array that convertsthe light into electrical signals used for imaging purposes. During datacollection, each pixel provides an electrical output signal proportionalto the absorbed photon flux. These output signals are then processed tocreate an image of the internal features of the subject (See M. Mazzilloet al., Silicon Photomultipliers for Nuclear Medical ImagingApplications, Proc of SPIE, Optical Sensors 2008, Vol. 7003 70030I-1).

Schottky photodiodes are majority carrier devices, and as such, they maybe preferred because they allow a faster timing response thanconventional p-n junctions. In Trench Sidewall Contact SchottkyPhotodiode and related method of fabrication, VA2009A000033, filed Jun.1, 2009 to M. C. Mazzillo, a vertical Schottky photodiode structure isdisclosed. The thickness of the depleted region of which is adjusted byvarying the depth of metal contact trenches in a lightly dopedsemiconductor epilayer, and even the thickness of the epilayer may bereduced to trim the sensitivity of the device to high wavelengthphotons.

Of course, single pixels or photo detector arrays, including Schottkyphotodiodes, can be used for detecting the radiation without using anyscintillator material, as, for example, in spectroscopic andastronomical imaging applications where the electromagnetic radiation tobe detected is in the range of sensitivity of the semiconductor used forthe photo sensor fabrication.

Charged Coupled Device (CCD) technology is commonly used for imagingapplications due to its relatively high quantum efficiency in thevisible band and low readout noise, even at relatively high scan rates.In recent years CMOS-Active Pixel Sensors (APS) have been developed andused for high speed imaging applications, for example, in adaptiveoptics, star trackers, and fast video-rate readout systems. Though CCDdevices exhibit better performance in terms of high fill factor and,consequently, high quantum efficiency and low noise, CMOS APS devicesmay be more used because of their superior response performances, lowfabrication cost, and easy foundry access (See G. Bonanno et al.,CMOS-APS for Astrophysical Applications, Memorie Salt 2003, Vol. 74, pp.800-803).

A great effort has been recently spent for the realization of SPAD(Single Photon Avalanche Diode) imaging arrays, which by exploiting thefaster time response (<100 ps) of photodiodes allow a three-dimensionalimaging of the objects by using time-of-flight techniques. The packagingof these devices and the integration of the detector with the ancillaryelectronics is a quite complex matter, and lately various techniqueshave been developed to realize compact and cheap “packaging” approaches.

In any case, large area and high fill factor arrays may be desired. Thepixels of which should be individually addressable by independentdriving and readout circuits. The larger the number of pixels in thearray the greater the dynamic range, and thus the more accurate thespatial information provided by the whole photo detector. Indeed, thelarger the size and the geometrical fill factor of the array, thegreater its sensitivity.

According to semiconductor device fabrication processes, chips are builtup in large numbers on a single large “wafer” of semiconductor material,typically silicon. The individual chips are patterned with small pads ofmetal (usually near their edges) for connections to leads of a metalframe. The chips are then cut out of the wafer bonded to a metal frameor carrier, and the pads are connected to the metal leads, typicallywith small wires (wire bonding). In case of a multi-pixel array, anindividual connection of each array pixel to an external circuitry maybe desired, and for arrays with a large number of pixels wire bondingmay become practically impossible because of packaging constraints.

A possible approach to overcome this problem may be to integrate thesensor and the electronics in a compatible technology process flow suchthat by integrating complex multiplexed electronics architectures, theoverall number of external leads may be made compatible with thepackaging constraints of evidence. However, this approach significantlyreduces the area occupancy ratio between sensing area and overall chiparea (geometrical fill factor) limiting the dynamic range and/orsensitivity (See Niclass et al., Design and Characterization of a CMOS3-D Image Sensor Based on Single Photon Avalanche Diodes, IEEE Journalof Solid-State Circuits, Vol. 40, No. 9, pp. 1847-1854, 2005). Moreover,it may not be possible to integrate the electronics and the photo sensorfabrication technologies in a monolithic device fabrication process.

Integration of a detector array with ancillary electronics may be madepossible by using more complex packaging techniques like “Flip Chip” or“Bridge” Bonding. Both contemplate bonding of two distinct chipsface-to-face and illuminating the detector array from the substrate side(so-called back illumination). In Flip Chips the semiconductor detectorcells are “bump-bonded” to readout circuits by defined arrays of indium(In) or solder bumps. Such a technique has been used, for example, forindium-gallium-arsenide (InGaAs) photodiodes grown epitaxially onindium-phosphide (InP) substrates. In this case, the substrate materialis relatively transparent at the wavelength where the epitaxially thickdetector is used so that the devices can be used in back-illuminatedmode without eliminating the substrate onto which they were formed.However, in other situations, for example, in homo-epitaxial devicesrealized on silicon or on silicon carbide, the substrate is opticallyopaque at wavelengths the detector has to be used, and therefore, it maybe desirable to remove the substrate leaving a detector structure thatmay be only a few microns thick. In such a “bump-bonding” process, asufficient mechanical sturdiness may be ensured by filling the spacesbetween the bumps before detector substrate removal. Concerns about thescalability of this type of process to large array sizes led to thedevelopment of an alternative process known as “bridge bonding” (See B.F. Aull et al., Geiger-Mode Avalanche Photodiodes for Three-DimensionalImaging, Lincoln Laboratory Journal, Vol. 13, No. 2, pp. 335-350, 2002).

In a “bridge bonding” process, the detector array and the electronicschips are epoxied together, and detector substrate removal is carriedout. Electrical connections are made last by etching vias between thephotodiodes and patterning metal connections in the vias. Successfuldevelopment of the bridge-bonding process required overcoming a numberof technological hurdles. First of all, the thinning must be uniform.Then curing of the epoxies used must not lead to destructive mechanicalstresses due to thermal-expansion coefficient mismatch betweensemiconductors and epoxies. The vies through the epoxy must have slopedsidewalls to allow good step coverage of the bridge metal. Moreoverbecause of the vias, most of the required photolithographic steps aredone on a non-planar surface, which raises some problems of non-uniformphoto resist thickness and exposure depth-of-focus issues. Finally,handling of the photo detector wafer should not lead to excessiveincreases in leakage current or dark count rate (See B. F. Aull et al.,Geiger-Mode Avalanche Photodiodes for Three-Dimensional Imaging, LincolnLaboratory Journal, Vol. 13, No. 2, pp. 335-350, 2002).

A possible way to front illuminate the detector for avoiding removal ofthe substrate is to use Through Silicon Vies (TSVs) for the integrationof the detector with the electronics. TSV is a vertical electricalconnection (via) passing completely through a silicon wafer or die, andat present, may potentially be the best technique of system integration.In this case, the detector die and the electronics chips are verticallystacked, and the contacts from the topside of the detector are realizedthrough vias to the backside of the die, then suitably soldered to thedriving and readout circuitry.

The through-via technology made remarkable advances in the latter halfof the 90 s, when important process technologies, such as deep siliconetching, wafer thinning, and wafer/chip bonding were developed. However,high cost of these techniques make them unsuited for low-end products(See K. Takahashi and M. Sekiguchi, Through Silicon Via and 3-DWafer/Chip Stacking Technology, 2006 Symposium on VLSI Circuits, pp.89-92).

SUMMARY OF THE INVENTION

Multiplexing of array pixel information to communicate with the externalworld through a limited number of leads of a semiconductor integratedsensor device, may be accomplished in an increasingly effective andefficient manner in the multi-pixel, photo detector array in accordancewith the present embodiments.

The sensing structure of the multi-pixel, photo detector array of thepresent disclosure may be a Schottky photodiode having a distributedanode contact buried in the bulk of a semiconductor layer, in the formof metal filled trenches and/or holes, uniformly spaced from one anotherand generally distributed at regular intervals within the respectivepixel sensing area. The Schottky photodiode structure may have anintegrated structure similar to the one disclosed in prior Italianpatent application No. VA2009A000033, of the same applicant, filed onJun. 1, 2009, the entire content of which is herewith incorporated byreference.

An important aspect of the novel multi-pixel, photo detector array isthat the parts or portions of the distributed anode contact of thedistinct elementary sensing structures of the multi-pixel array may beconnected to a common anode current collecting grid contact, patternedin a front metallization layer in a way as to have a reduced foot-printover the whole front area of the photo detector array, and theelementary sensing structures of the multi-pixel array may share acommon cathode contact.

Thus, such a two-terminal, multi-pixel, photo detector array maymultiplex the individual pixel information through a common anode and acommon cathode contact.

Discrimination among the information coming from the array pixels interms of wavelength of the incident radiation may be possible byadapting the depth and the spacing of the Schottky metal filled trenchesand/or holes of the distributed anode contact in the semiconductor ofeach elementary Schottky diode of the array to produce a unique depth(thickness) of the depletion layer and/or a unique pinch-off condition,at least one of which is, in general, significantly different from thatof any other Schottky diode of the array at a given wavelength of theincident photons.

The effectiveness of such a modulation of these fundamentally geometricparameters in determining the resulting depth of the profile (thickness)of the depletion layer (i.e. of the sensible photon absorption region inthe semiconductor) and the pinch-off condition of depletion regionsproduced around the uniformly spaced Schottky metal/semiconductor trenchcontacts of the each pixel photodiode (i.e. corresponding to its maximumdetection efficiency condition) has been found to be substantiallyequivalent for a variety of commonly used semiconductors. By properlydesigning the geometrical aspect ratio and spacing of the metal filledtrenches, it may be retained also at very low reverse bias condition ofthe diodes and even at null bias, that is under purely photovoltaicregime.

The array may be particularly suited for imaging applications withmonochromatic or substantially so photons of wavelength falling in thelow energy (high wavelength) end of the range of sensitivity of thesemiconductor used for the fabrication of the multi-pixel array sensor.

It may be well known that in any semiconductor material the photonabsorption coefficient decreases with the wavelength (See, for example,H. Y. Cha and P. M. Sandvik, Electrical and Optical Modeling of 4H-SiCAvalanche Photodiodes, Japanese Journal of Applied Physics, Vol. 47, No.7, pp. 5423-5425, 2008, and S. M. Sze, Semiconductor Devices Physics andTechnology, New York: Wiley, 1985). At 350 nm, for instance, the“absorption length” in 4H-SiC is about 35 μm, meaning that 67% of theincident optical flux at this wavelength is absorbed in an active layer(a depleted semiconductor region) 35 μm thick (See J. Hu et al., 4H-SiCVisible-Blind Single-Photon Avalanche Diode for Ultraviolet Detection at280 and 350 nm, IEEE Transactions on Electron Devices, Vol. 55, No. 8,pp. 1977-1982, 2008). This means that it may be generally necessary toincrease as much as required or possible, the thickness of the activeregion (i.e. the depth of the depleted region in the semiconductor) toenhance absorption, and consequently, photon detection effectiveness atthe long wavelength end of the sensitivity range of the semiconductor.The ability to grow lightly doped semiconductor epilayers of lowdefectivity and of relatively large thickness, in the order of severaltens of μm, and the ability to etch deep trenches with a high aspectratio in a semiconductor epilayer and to fill them with a high Schottkybarrier metal, may make it relatively easy to adapt the geometricparameters that determine the operative thickness (depth) of thedepletion layer amongst the Schottky photodiode structures of individualarray pixels (i.e. designing the geometry of the photodiode of eachpixel for a unique response to illumination with photons of a givenwavelength).

A relatively light doping of a semiconductor epilayer, in which thearray of Schottky photodiodes is formed, may promote the creation ofwide depleted regions already at very low reverse bias condition of thediodes. By suitably designing the distance among adjacent trenches, thepinch-off condition, corresponding to the condition of fullest detectioneffectiveness, may be achieved with a relatively low reverse bias andmay be even in photovoltaic regime (0 V bias of the photodiode).

Exemplary ways of how a multi-pixel, photo detector array of theapplicant may be fabricated with ordinary and relatively inexpensiveintegration process techniques will be illustrated in the ensuingdescription, without intending to limit the breath of protection to theillustrated embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross section of two adjacent pixels of a photodetector array according to the present embodiments.

FIG. 2 is a diagram of an exemplary layout of a 4×4 multi-pixel photodetector array having different pixel structures, as shown in FIG. 1.

FIGS. 3-19 schematically illustrate a sequence of steps of a fabricationprocess of the multi-pixel photo detector array according to the presentembodiments.

DETAILED. DESCRIPTION OF THE PREFERRED EMBODIMENTS

The novel architecture of multi-pixel photo detector array of Schottkyphotodiodes for imaging applications with substantially monochromaticradiations of wavelength comprised in a high wavelength end portion ofthe sensitivity range of the semiconductor used is schematicallydepicted in FIG. 1.

According to the considered exemplary embodiment, FIG. 1 shows avertical cross-sectional view of two adjacent pixels of the array. Alikeare the two pixels, 1^(ST) _(—) PIXEL and 2^(ND) _(—) PIXEL, shown inthe descriptive fragment of the cross-sectional view of FIG. 1. Thepixel photodiodes are formed in a lightly doped, n⁻⁻ type layer 14,epitaxially grown over a relatively thin moderately doped, n⁻ typebuffer layer 12, on a heavily doped, n⁺ type mono crystalline substrate10. The cathode contact-terminal pad of the array is common to all thepixels and may be formed by a commonly, not patterned, metal layer or,more typically, a multilayer metal stack, 22, 24 and 26, deposited overthe rear surface of the mono crystalline semiconductor wafer ofsubstrate 10.

One pixel photodiode 1^(ST) _(—) PIXEL has trenches filled with highbarrier Schottky metal 16′, usually tungsten or aluminum, all ofidentical depth that may be uniformly spaced from one another throughoutthe active area projection of the diode, and different, from the depthand uniform spacing of the metal filled trenches 16″ of the otherphotodiode 2^(ND) _(—) PIXEL. As a consequence, the depletion layers'profiles of the two photodiodes, traced with a dotted line, aredifferent and substantially the useful photon absorption region in thesemiconductor epilayer 14, assumes different depths (thicknesses),namely: T₁ and T₂, respectively, in the two array pixels.

In the ensuing description, it is presumed that the photons impinge ononly one pixel of the array at the time. That is, only one pixel at thetime is impinged by photons. In practice, the flux of photons has across-section sufficiently small to excite a single pixel.

For example, in an array of scintillation detectors an elementaryparticle (a gamma ray) is absorbed by only one detector at the time.When the detector absorbs an elementary particle, it emits a photon. Byassociating a pixel of the array to each scintillation detector, it maybe possible to identify the detector that has captured the elementaryparticle by recognizing, in terms of relative intensity of response, thepixel that captured the emitted photon.

If a flux of relatively long wavelength monochromatic photons impingeson the active surface area of these array pixels, the number of pairs ofelectrical carriers that are generated in the depleted layer of eachpixel upon absorption of photons will be different because the degree ofabsorption depends from the thickness of the sensitive depletion regionin the semiconductor. In other words, the number of pairs of electricalcarriers may be different from different absorption characteristics ofeach photodiode of the array.

Therefore, spatial information may be provided for any specificwavelength of the flux of energetic photons by the differentelectro-optical response of each pixel to an impinging monochromaticphoton flux, and this may relieve the need of providing an independentoutput for each pixel of the array, as in comparable photo detectorarrays for imaging of the prior art.

In other words, the pixels connected to a same anode line generatedifferent photocurrents in response to the absorption of a photon.Therefore, by reading the current collected on the anode line, it may bepossible to identify the pixel that has captured the photon.

The distributed anode trench contacts 16′, 16″, etc. of the Schottkyphotodiodes of the pixels 1^(ST) _(—) PIXEL, 2^(ND) _(—) PIXEL, etc. ofthe photo detector array can be connected to a common patterned metalanode collecting grid 18. This is schematically depicted in FIG. 2 foran exemplary 4×4 photo detector array, wherein two parallel anode metalbuses, to which the distributed anode trench contacts 16′, 16″, etc. ofthe Schottky photodiodes of the pixels are connected, lead to a uniqueanode terminal pad of the two-terminal, multi-pixel photo detector arrayof this disclosure.

The novel two terminal array architecture of this disclosure may bealternatively formed even with both the anode and the cathode gridcontacts patterned in the same front metallization layer, for example,in the form of two interleaved multi-bus collector/distributor, metalpatterned structures. In this case, the fabrication process flowprovides for bringing to the front surface, the common cathode contactof each pixel photodiode through the lightly doped epilayer withoutcreating spurious rectifying metal/semiconductor contacts therewith.

The possibility of forming a multi-pixel, photo detector array with one(two-terminal) output, or even with two outputs (three or fourterminals), by multiplexing all the array pixels or the pixels of twodistinct groups of array pixels, respectively, may offer the possibilityof generating simultaneous images for two distinct wavelength,monochromatic photon fluxes of the same object or source surface. Thegroups of pixels may be, for example, a first order or group of pixelswith individually differentiated response characteristics to photons ofa first wavelength, and a second order or group of pixels withindividually differentiated response characteristics to photons of asecond wavelength sufficiently spaced from the first wavelength thoughfalling both in the high wavelength portion of the spectrum ofsensitivity of the semiconductor used. The multiplexing of the spatialinformation, besides reducing the number of leads, also entails a muchsimpler driving and readout circuitry of the imaging device, and thissimplification may favor integration of the biasing and readoutcircuitries on the sensor chip, overcoming packaging problems andrelated costs of the imaging device.

Of course, in case of an array contemplating grouping of pixels in twodistinct orders for simultaneous imaging with two distinct monochromaticphoton fluxes of different wavelength, the pixels of the two orders maybe similarly distributed over the sensor area (e.g. in pairs of adjacentpixels of the two orders) according to a pattern and a duplication ofthe respective common anode current collecting grid contact, patternedin the front metallization layer, one for each group of pixels.

In any case, the novel sensor device may offer a high fill factorbecause of the reduction of blind area not requiring a burdensomeplurality of patterned metal lines for providing an independent anodeoutput for each pixel. The advantage increases for arrays with a largenumber of pixels.

A light doping of the epitaxial layer allows reaching of the pinch offcondition among the depletion regions that are created around eachtrench anode contact at relatively low reverse bias, by suitablydesigning the distance of separation (spacing) among adjacentmetal-filled trenches of each pixel. Therefore, properly designeddistributed anode trench contacts of the array pixels may work atfullest detection efficiency for high wavelength (low energy) photonsunder an extremely low reverse bias, and even at null bias (0V bias), inwhat may be defined purely photovoltaic regime.

The distributed Schottky metal electrode of each photodiode (typicallytungsten or aluminum) set in uniformly spaced deep trenches reducesunwanted cross talk effects among adjacent array pixels, and amongadjacent parts of the distributed Schottky contact of each elementaryphotodiode (pixel) reducing the probability that an absorbed photon maycreate two or more carrier pairs in the depleted semiconductor region.Thus a spurious gain enhancement may be produced that would in turncause of error in the estimation of the photonic flux absorbed in thearray pixel.

In the sequence of sectional fragments, from FIG. 3 to FIG. 19, the mainsteps of an exemplary fabrication process flow are illustrated anddescribed herein below using the reference numerals used for describingthe functional multi-pixel photo detector array structure of FIG. 1. Theexemplary process flow may be followed for integrating a multi-pixelphoto detector array of this disclosure on an N-type silicon, substrate10, over which, according to common practice in the art, a first, thinN-type buffer layer 12 and a second, thick N-type layer 14 areepitaxially grown, as depicted in FIG. 3.

The dopant concentration of the substrate 10 may be in the range 1E18 to1E19 atoms/cm³, and the dopant concentrations of the thin epitaxialbuffer layer 12 and of the thick epitaxial layer 14 may be in the ranges1E17 to 1E18 atoms/cm³ and 1E13 to 1E15 atoms/cm³, respectively. Thesubstrate 10 may have a thickness in the range 350-400 μm, the thinepitaxial buffer layer 12 a thickness in the range 0.5-1 μm, and thelightly doped thick epitaxial layer 14 a thickness in the range 3-150μm.

The heavily doped substrate 10 may provide for establishing anon-rectifying (ohmic) cathode contact, common for all the array pixels,with an appropriate metallization of the whole rear surface of thesubstrate, connectable to a photo detector circuitry that can be on thesame chip or even external to the photo detector array chip.

The thin epitaxial layer 12 acts as buffer layer on which the thicklightly doped epilayer 14 may be more reliably grown. The light dopingof the thick N⁻⁻ epitaxial layer 14 promotes the creation of relativelywide space charge (i.e. depleted) regions that enhance collectionefficiency of photo generated carriers in the thick N⁻⁻ epilayer, evenunder a relatively low reverse bias. An ordinary zero-layer photo maskmay be used for the definition of appropriate alignment marks (not shownin the drawings).

Then, as depicted in FIG. 4, a thick layer acting as sacrificial layer,typically a dielectric (e.g. SiO₂ or Si₃N₄) on silicon or a metal (e.g.Ni, Cr, Al) on 4H-SiC is deposited (or sputtered in the case of metal)on the wafer. The thickness of this layer may be in the range 1-10 μm,depending on the depth of the trenches to be etched in the semiconductorepilayer 14.

By a common lithography mask of photoresist, equally spaced trenches aredefined along one side of one of the two adjacent photodiodes while inthis phase the sacrificial layer onto the surface of the secondphotodiode remains protected by the photoresist, as depicted in FIG. 5.Using a calibrated etch process (dry or wet), the sacrificial layer isremoved from areas where distributed anode trench contacts of the firstphotodiode will be etched, as shown in FIG. 6.

After removing the photoresist mask, with a specific and calibrated dryetch, deep trenches are cut into the semiconductor, in the area of thefirst photodiode. During this step, the sacrificial layer acts as hardmask protecting from the etch the semiconductor epilayer 14 in the areaamong the uniformly spaced trenches of the first photodiode being etchedand the wholly the epilayer 14 in what will be the area of the adjacentsecond photodiode. The depth of etched trenches in the first photodiode,as exemplarily shown in FIG. 7, may be at most equal to the thickness ofthe lightly doped second epitaxial layer 14. In any case, the trenchesto be filled with the selected high barrier metal to constitute thedistributed anode contact of the Schottky photodiode, generally must notreach as far down as to cause a short circuit with the ohmic cathodecontact on the bottom of the wafer by a common metallization. Afterhaving removed the residual sacrificial layer typically with a wet etch,a metal (usually tungsten) 16′ is deposited with a CVD process tocompletely fill the trenches followed by an etch back to remove themetal from the planar surface, as shown in FIG. 8.

The whole sequence of steps, from the sacrificial layer deposition asfar as to the filling of the trenches with the selected high barriermetal, is repeated to form the distributed anode trench contacts 16″ ofthe second photodiode, as shown in the sequence from FIG. 9 to FIG. 13.The sequence of steps is repeated for as many times as the number ofpixels of distinct sensitivity of the array, every time with differentgeometric parameters of depth of the metal filled trenches and ofspacing among the trenches, according to a pre-established distributionpattern of pixels of different characteristics, to form a multi-pixel,photo detector array of appropriate size for producing a singlemonochromatic image or simultaneous monochromatic images of illuminatedobjects or of a source of monochromatic photons and similar imagingapplications with photon fluxes of wavelength falling in thehigh-wavelength end portion of the sensitivity range of thesemiconductor used, where conditions exist for implementing a fineadaptation of the absorption characteristics of the array pixels to bediscriminately read under an impinging flux of monochromatic photons.Notably, in case of a wavelength falling in the short wavelength regionof the spectrum, absorption in the depleted region of a semiconductordevice of the impinging radiation may be complete in one or few micronsof travel, and may be impossible to achieve a sufficient discriminationof photoelectric conversion yields of the array pixels.

Equipotentiality of the Schottky metal filler of the trenches ofdistributed anode contact of the pixel photodiodes of the array isprovided by an anode current collecting metal grid 18, for example, anAl—Si—Cu ternary mixture/alloy sputtered over the front surface andlithographically defined to form capping lines over and in electricalcontact with the Schottky metal filler 16′ and 16″ of the trenches, asdepicted in FIG. 14 and FIG. 15. The Al—Si—Cu ternary mixture/alloy canbe sputtered on the wafer for a final thickness that may be in the rangeof 1-3 μm, and by ordinary photolithography and calibrated wet etchprocess, the metal is removed from the planar surface of the wafer, inthe areas of separation among the trenches, in the pixels of the array.

A common dielectric passivating layer 20, for example, a partly oxidizedsilicon nitride layer may then be deposited, as shown in FIG. 16, tocoat the patterned anode contact metal grid structure of the multi-pixelarray. The thickness may be in the range of 0.3-1 μm. With a furtherphotolithography and dry etch sequence depicted in FIG. 17 and FIG. 18,the passivating layer 20 is patterned to entirely encapsulate thepatterned metal parts of the anode contact metal grid structure. Asintering treatment at a relatively low temperature in hydrogen can becarried out to reduce surface electronic states concentration, andconsequently, the SRH generation rate and the leakage current of thephotodiodes.

Finally, a metallic multilayer stack is deposited on the back surface ofthe wafer to form a bottom ohmic cathode contact common to all the arrayphotodiodes that are thus electrically connected in parallel to thetwo-terminals of the array. Referring to FIG. 19, three differentmetallic layers can be sputtered in sequence on the back of the wafer.The first layer 22 may be, as usual, titanium, for providing aninterface barrier with the semiconductor heavily doped substrate 10. Thesecond layer 24 may be nickel, or more infrequently platinum, and thetopping layer 26 may be a flash layer of gold for promoting lowresistance wire bonding. The thickness of the layer 22 can be in therange 500-1000 Å, that of the second layer 24 in the range 2000-5000 Å,and that of the layer 26 can be in the range 200-500 Å.

The different sensitivities of the pixels intended in terms of differentphotoelectric conversion yields of the incident monochromatic photonflux, determined by the different thickness of the depletion layer thatis created in each array pixel, provides the information about the pointof absorption of the photons over the sensible area of the multi-pixelarray. Due to the multiplexed anode contact configuration, the pixelphotodiodes of the array are electrically in parallel and work at thesame reverse bias condition. Therefore, the different response of eachpixel to the incident photon flux is exclusively due to the differentdepletion layer thickness and not to different bias conditions.

Different semiconductors, such as for example silicon, silicon carbideand gallium nitride, can be used for making the solid-state photodetector array of this disclosure, while metals like tungsten, aluminum,and nickel may be used to fill the trenches for forming the distributedSchottky trench anode contact of the pixel photodiodes.

1. A method of making a multi-pixel, photo detector array of Schottkyphotodiodes on a semiconductor substrate having a dopant concentrationbetween 10¹⁸ and 10¹⁹ atoms/cm³, comprising: a) growing a firstrelatively thin epitaxial layer with a dopant concentration between 10¹⁷and 10¹⁸ atoms/cm³ of a same conductivity type as a dopant of thesubstrate, on a front side thereof; b) growing a second relatively thickepitaxial layer with a dopant concentration between 10¹³ and 10¹⁵atoms/cm³ of same conductivity type as the dopant of the substrate and athickness between 3 and 150 μm, over the first relatively thin epitaxiallayer; c) depositing a sacrificial metal hardmask layer; d) defining anarray of uniformly spaced parallel openings through the sacrificialmetal hardmask layer by a first lithography step of a given spacing andwidth of the uniformly spaced parallel openings; e) anisotropicallyetching the second relatively thick epitaxial layer through theuniformly spaced parallel openings of the metal hardmask layer, formingtrenches of a given depth, and removing residual metal of the metalhardmask layer; f) depositing a metal layer of Schottky contact by aconformal metal deposition technique for completely filling thetrenches, and etching the deposited metal layer for removing it from aplanar front surface; g) repeating steps c) d) e) and f) for each otherpixel of the array for as many different depths of the metal filledtrench contacts as the number of pixels of the array of respectivespacing and width of the metal filled trenches; h) depositing a metallayer on the planer front surface in contact with the Schottky contactmetal filler of all the trenches of all the array pixels; i) defining acommon anode current distributor grid from the deposited metal layer forconnecting, in common, the Schottky contact metal filler of all thetrenches and forming a connectable anode terminal pad, by a secondlithography step and selective wet etch process; j) depositing apassivating layer conformally over the planar front surface for coatingthe defined parts of the metal layer; k) removing the passivating layerin areas of separation among coated parallel stripes of the common anodecurrent distributor defined over the metal filled trenches by a thirdlithography step and dry etch process; and l) depositing a metallizationlayer over the rear surface of the semiconductor substrate to define aconnectable cathode terminal.
 2. The method of claim 1, furthercomprising performing a sintering process for reducing a concentrationof surface electronic states.
 3. The method of claim 1, wherein thesubstrate and the first relatively thin epitaxial layer and secondrelatively thick epitaxial layers comprise nitrogen doped siliconcarbide having n-type conductivity.
 4. The method of claim 1, whereinthe first relatively thin epitaxial layer has thickness between 0.5 and1.0 μm.
 5. The method of claim 1, wherein the Schottky contact metalfilling the trenches comprises at least one of tungsten and aluminum. 6.The method of claim 1, wherein the rear metallization layer comprises afirst barrier layer of titanium, an intermediate layer of at least oneof nickel and platinum, and a top layer of gold.
 7. A method of making amulti-pixel, photo detector array of Schottky photodiodes on asemiconductor substrate having a dopant concentration between 10¹⁸ and10¹⁹ atoms/cm³, comprising: a) forming a first relatively thin epitaxiallayer with a dopant concentration between 10¹⁷ and 10¹⁸ atoms/cm³ of asame conductivity type as a dopant of the substrate, on a front sidethereof; b) forming a second relatively thick epitaxial layer with adopant concentration between 10¹³ and 10¹⁵ atoms/cm³ of sameconductivity type as the dopant of the substrate and a thickness between3 and 150 μm, over the first relatively thin epitaxial layer; c) forminga sacrificial mask layer; d) forming an array of uniformly spacedparallel openings through the sacrificial mask layer by a firstlithography step of a given spacing and width of the uniformly spacedparallel openings; e) etching the second relatively thick epitaxiallayer through the uniformly spaced parallel openings of the mask layer,forming trenches of a given depth, and removing residual portions of themask layer; f) forming a metal layer of Schottky contact filling thetrenches; g) repeating steps c) d) e) and f) for each other pixel of thearray for as many different depths of the metal filled trench contactsas the number of pixels of the array of respective spacing and width ofthe metal filled trenches; h) forming a metal layer on the planer frontsurface in contact with the Schottky contact metal filler of thetrenches of all the array pixels; i) forming a common anode currentdistributor grid for connecting, in common, the Schottky contact metalfiller of the trenches; j) forming a passivating layer over the planarfront surface for coating the defined parts of the metal layer; k)removing the passivating layer in areas of separation among coatedparallel stripes of the common anode current distributor grid; and l)forming a metallization layer over the rear surface of the semiconductorsubstrate to define a connectable cathode terminal.
 8. The method ofclaim 7, further comprising performing a sintering process for reducinga concentration of surface electronic states.
 9. The method of claim 7,wherein the substrate and the first relatively thin epitaxial layer andsecond relatively thick epitaxial layers comprise nitrogen doped siliconcarbide having n-type conductivity.
 10. The method of claim 7, whereinthe first relatively thin epitaxial layer has thickness between 0.5 and1.0 μm.
 11. The method of claim 7, wherein the Schottky contact metalfilling the trenches comprises at least one of tungsten and aluminum.12. The method of claim 7, wherein the rear metallization layercomprises a first barrier layer of titanium, an intermediate layer of atleast one of nickel and platinum, and a top layer of gold.
 13. A methodof making a multi-pixel photo detector array comprising: forming aplurality of Schottky photodiodes on a semiconductor substrate, eachSchottky photodiode comprising a plurality of metal filled anodecontacts having a depth different than a depth of metal filled anodecontacts of at least one other Schottky photodiode; forming a commoncathode coupled to the plurality of Schottky photodiodes; and forming acommon anode collecting grid coupled to the plurality of metal filledanode contacts.
 14. The method of claim 13, wherein the common cathodecontact comprises a non-patterned rear metallization layer.
 15. Themethod of claim 13, wherein each of the plurality of Schottky pixelphotodiodes has a different depletion layer depth.
 16. The method ofclaim 13, wherein the plurality of Schottky metal filled anode contactsfor a given one of the plurality of Schottky pixel photodiodes has auniform spacing therebetween.